Rousseff does not personally face corruption charges, but is accused of breaking budget accounting rules ahead of her 2014 re-election campaign.

Brazil's impeachment trial

- The Senate has 180 days maximum to conduct hearings, which will be monitored by the chief justice of the Supreme Federal Tribunal - the country's top court.

- If two-thirds of 81 senators then support impeachment, she will be permanently removed from office.

- She will have to step down immediately in case of a "yes" vote and will be banned from public office for eight years.

- The vice president is in charge during her suspension and will complete rest of the presidential tenure until 2018 in case of a "yes" vote.

- If the Senate votes against the impeachment or if no decision is taken within the stipulated 180 days, Rousseff's suspension will end and she will return as president.

Franciso Dominguez of Middlesex University said these financial maneuvering have been practiced by many governments in the past.

"Many of the parties that actually voted for impeachment now, supported and approved these financial maneuverings by previous administrations, including president Fernando Cardoso and Rousseff's predecessor Luiz Inácio Lula da Silva," he said.

A two-thirds majority will be needed to remove Rousseff permanently at the conclusion of the trial, with Temer remaining president until 2018 elections.

As senators debated in Brasilia, where a metal fence was erected to keep apart rival protests, about 6,000 backers of impeachment chanted "Out with Dilma" while police used pepper spray to disperse gangs of Rousseff supporters, who hurled flares back. One person was arrested for inciting violence.

In Sao Paulo, the economic capital of Brazil, people on both sides of the debate also took to the streets.

"Some said the president should be impeached, that an investigation should be launched into whether her government and Rousseff did anything to hurt this country's economy," Al Jazeera's Teresa Bo reported.

"Others said historical injustices are being committed and that an innocent woman was being put on trial."

Dominguez said he expected Brazilians opposed to impeachment to continue protesting.

"I can foresee the state exploding," he said. "The country is going to witness a huge wave of discontent, possibly strike of every kind and so on because... the process is seen as so unfair," he said.

Rousseff took office in 2011 as the country's first female president. She faces impeachment over accusations of tampering with figures to disguise the size of Brazil's budget deficit during her 2014 re-election campaign.

She has denied any wrongdoing, and cast the efforts to remove her as a coup.